For many years diatomite products have been used as filter aids in a wide variety of filtration processes. Diatomite filter aids are normally marketed as three different types: natural diatomite, calcined diatomite and flux calcined diatomite. The natural diatomite is diatomite which has been mined, dried, crushed and classified as by air separating or screening into diatomite granules or particles of specified size ranges. Calcined diatomite is diatomite which is mined, dried, granulated and passed through a kiln which is operated at temperature on the order of about 1800.degree. F. to 2400.degree. F. (980.degree. C. to 1315.degree. C.). The calcination causes the diatomite particles to shrink and harden and, to a certain extent, to agglomerate themselves into larger clusters. Calcination is enhanced by the use of fluxes such as sodium carbonate mixed through the diatomite powder prior to its being passed through the kiln. In general, the natural diatomites are used to provide high clarity of liquid filtrates, but have the disadvantage of having low flow rates of filtrate through the diatomite filter aid bed. The calcined materials provide progressively higher flow rates but result in lesser degrees of clarification of the filtrate per pass of filtrate through the filter aid bed.
In order to obtain a sufficient degree of calcination for calcined diatomites, considerable amounts of thermal energy must be used to maintain the kilns at the appropriate temperature during the prolonged period of approximately 30 to 90 minutes which it takes for the diatomite to pass through the kiln and become thoroughly calcined. This, of course, results in a large quantity of heat being required per individual unit of diatomite passed through the kiln. Researchers in the prior art have investigated various ways of reducing this per unit requirement of heat. One such approach was described by R. G. Riede in U.S. Pat. No. 3,013,981. In Riede's process, approximately 10 to 100 percent by weight of water or other liquid binder was added to diatomite. The two were mixed for a prolonged period in order to plasticize the mixture. A flux in the form of a dry powder or a liquid solution could also be added to the mixture of diatomite and liquid binder. Thereafter the material in the form of randomly sized agglomerated particles was passed into the kiln for calcination. Calcining times continued to be on the order of 30 to 90 minutes, but the Riede process of forming random sized agglomerates permitted kiln calcination temperatures to be reduced to temperatures on the order of about 1700.degree. F. to 1850.degree. F. (925.degree. C. to 1000.degree. C.). While the Riede process was used for some time for "flux calcining" because it simplified the incorporation of flux (dissolved in the water) into the diatomite, it was gradually displaced by calcination processes which utilized dry flux addition to diatomite without the presence of a binder. While these processes required somewhat greater kiln times to effect calcination, the economics as compared to the water-addition systems were more favorable with the lower energy costs then prevailing.
Wetting of diatomite has also been mentioned in U.S. Pat. No. 2,693,456 to J. E. Fennell. In the Fennell process 20 to 50 percent water is incorporated into dry diatomite powder along with flux prior to conventional flux calcination of the diatomite powder. The water presence is said to improve the filtration rate and color properties of the end product. The calcination process is described as conducted under ordinary time and temperature conditions, with no improvement in either mentioned.
In more recent years, energy costs have climbed immensely. It has therefore become imperative to develop a process for the calcination of diatomite which will result in substantial reductions in the usage of thermal energy per unit of diatomite processed while yet providing a finished product having the required and desirable properties of the prior art diatomites. The Riede process has been considered but has not been determined to be adequate, for the small amount of reduction in kiln temperature available through the Riede process does not offset the disadvantage that the Riede process still requires prolonged kiln processing time. It would therefore be highly desirable to have a process which would permit a significant reduction in the amount of time required to effect calcination of diatomite and which would provide for a major reduction in the amount of thermal energy required for complete calcination.